206 
THE GEOLOGIST. 
M. Descloizeaux, a distinguished mineralogist of Paris, who won 
himself some reputation by his geological mission to Iceland, has been 
studying for a long time the optical properties of crystals. He has 
lately addressed to the Academy of Sciences a new memoir on this 
subject, and hopes soon to have completed liis numerous observations 
on double refraction, principal axes of refraction, their number, posi- 
tion, and relations to the optical axes, the laws of dispersion, &c., in 
transparent minerals. It appears that the number of transparent 
substances in tlie mineral world, including those which are transparent 
enough when taken in thin laminae to give passage to a ray of light, 
is about 180. Of these, 106, of wliich eighty-one have one axis, and 
eighty-five two axes, have been completely studied by M. Descloizeaux. 
Twelve alone remain of which the optical co-efficients are not yet satis- 
factorily determined. 
We have alluded in former papers to the action of metamorphism 
by eruptive rocks on combustibles (lignite, coal, &c.), also to the 
action of lava and trap-rocks on limestones, argillaceous strata, and 
sandstones. We have now before us a new memoir by M. Delesse, in 
which granite is the eruptive rock under consideration. 
When metamorphism is studied with respect to granite-rocks it is 
seen that their effects differ notably from those produced by the 
different varieties of trap. Tlie following are the characters presented 
by strata that have undergone metamorphism by contact with 
granite. If the rock acted upon be limestone, it often happens that it 
has not been modified at all, even where it has been penetrated, or 
even where it has been covered over, by granite. The glauconite, so 
frequent in calcareous strata, remains also unaltered. More frequently, 
however, the structure of the calcareous rock has become crystalline, 
and of a paler coloni", having passed into saccharoid limestone. If the 
limestone be argillaceous, it has become very compact and lithoid, but 
not silicified. In some cases it has become cavernous, but has not 
passed into dolomite ; oftentimes, indeed, it contains less magnesia 
where it is in immediate contact with the upheaved granite. Among 
the minerals that have been developed in limestone under these cir- 
cumstances, we must name more especially spathic carbonates, quartz, 
and minerals common in metallic lodes. The latter form serpentine 
veins in the metamorphosed rock, or line some of its cavities. 
When a siliceous rock has been upheaved by granite, we observe 
that the metamorphism has been equally irregular ; sometimes com- 
pletely null ; sometimes so complete that the wliole rock has been 
transformed into transparent quartz. Quartz must in this case be 
noted as the most important mineral developed in immediate contact 
■with granite. Next comes sulphate of baryta, with which the quartz 
is often associated, fluor-spar, and the minerals of metallic lodes. 
When the rock modified by gi-anite is argillaceous, its structure has 
become schistose or lithoid. In some cases this structure approaches 
to that of jasper ; but it has not been observed to have taken a 
viti'eous aspect (as if quartz had been formed). When the argillaceous 
